The present invention relates to a method and a device for embossing plies of web material, particularly, but not exclusively, plies of paper, tissue paper, and the like, for the production of articles made from multi-ply web material and consisting of two or more plies which are embossed and joined together.
The invention also relates to a manufactured article consisting of two or more plies which are embossed and joined together.
In the production of this type of manufactured article, it is necessary to meet various requirements relating to the production, appearance and functionality of the end product, which requirements sometimes conflict with each other. For example, one of the problems which arises in the production of embossed web materials wound in rolls consists in the tendency of the individual turns of the material to become nested in each other. This creates a problem in respect of production and also a drawback in the finished product, in that the presence of embossing nested in adjacent turns in the finished roll is perceived by the user as a defect in the end product.
Furthermore, one of the requirements which the embossing process must meet is that of obtaining a high volume of the end product. It is therefore desirable to carry out embossing which, in a way compatible with the other production requirements, imparts a high volume to the finished article.
The embossed sheet web material is used for the production of kitchen towels, toilet paper, serviettes, paper tissues, and the like. In general, this web material consists of two or more plies which are usually embossed separately from each other and subsequently joined together, usually by means of an adhesive or also by other methods, such as ply-bonding. Sheet products having a particular softness and thickness, and good liquid-absorption characteristics, are produced in this way.
The embossing and the joining of two or more plies is normally carried out by one of two methods, called xe2x80x9ctip-to-tipxe2x80x9d and xe2x80x9cnestedxe2x80x9d embossing. In the first case, two plies of embossed material are joined by lamination between two embossing cylinders with counter-rotating parallel shafts. The two cylinders are provided with points which match each other, at least partially, in a laminating gap formed at the point of tangency between the two cylinders. An adhesive is applied to the protuberances of one of the two embossed plies to provide permanent joining to the other ply at the positions of the protuberances of the other ply in the areas in which the points of the two embossing cylinders coincide with each other. Examples of embossing machines of this type are described in U.S. Pat. No. 3,414,459, U.S. Pat. No. 4,978,565, U.S. Pat. No. 5,173,351, U.S. Pat. No. 5,096,527, U.S. Pat. No. 3,961,119, WO-A-9720687, WO-A-9720688, WO-A-9720689.
In other devices, the plies are joined in such a way that the protuberances of one ply are nested in the cavities between adjacent protuberances of the other ply. In this case, the two embossing cylinders are not pressed against each other at the positions of the corresponding points, and the two plies are joined together by laminating with a pressure roller which interacts with the surface of one of the embossing cylinders, on which both plies are supported as they emerge from the gap between the embossing cylinders.
Examples of this type of embossing are described and illustrated in GB-A-1,225,440 and U.S. Pat. No. 3,694,300.
Normally, the embossing, whether of the tip-to-tip or the nested type, consists of a geometrical and uniform distribution of protuberances, typically of truncated conical or truncated pyramidal shape, over the two plies. To obtain a product with a more attractive appearance, systems in which protuberances of various shapes are combined with each other to obtain a particular design have been conceived. For example, U.S. Pat. No. 4,320,162 describes an embossing system in which embossing, consisting of a geometrical and uniform distribution of small protuberances arranged with a high density, forming a fine background embossing, combined with a low-density distribution of protuberances of complex shape and of larger size, forming a decorative motif, is produced on each of the two plies. A similar product is described in WO-A-9618771.
A disadvantage of this system consists in the fact that when it is desired to change the decorative motif it is necessary to make a new pair of embossing cylinders, or at least one new embossing cylinder, which means that not only the whole of the decorative motif but also all the points of the cylinder producing the background embossing have to be formed again.
The object of the present invention is to provide a method and a device for producing an embossed multi-ply web material of particularly good quality. More particularly, an object of the present invention is to provide a method and a device which can be used to obtain a product of high volume.
A further object of the present invention is to provide a method and a device which can be used to obtain a product which, when wound in a roll, does not give rise to the interpenetration of the embossed design between a number of overlapping turns.
Yet another object of the present invention is to provide a method and a device which can be used to obtain an article in which the two or more embossed plies are joined together by an adhesive, and in which the quantity of adhesive required is relatively limited, in order to avoid excessive stiffening of the end product.
Yet a further object of the present invention is to provide a product which is particularly soft and plump to the touch.
These objects and advantages, and others, which will be evident to persons skilled in the art from a reading of the following text, are achieved essentially by means of an embossing device comprising, in combination:
a first embossing unit with a first embossing cylinder provided with a first set of points and interacting with a first pressure cylinder for forming, on a first ply, a first set of protuberances having small sizes and a high density;
a second embossing unit with a second embossing cylinder provided with a second set of points and interacting with a second pressure cylinder and a laminating cylinder, for forming, on a second ply, a second set of protuberances having larger sizes and a lower density than the protuberances of the first set;
an adhesive dispenser for applying an adhesive to one of said plies before laminating.
In practice, the second pressure cylinder has a yielding cylindrical surface, and the laminating cylinder has a cylindrical surface of greater hardness than the surface of the second pressure cylinder. Thus the first ply does not undergo a second embossing, but only a flattening of the first embossing where it meets the points of the second embossing.
The points of the first embossing cylinder have a greater density and smaller size than the points of the second embossing cylinder.
The invention also relates to a method for the production of a multi-ply web material, consisting of at least two embossed plies joined together, comprising the steps of:
embossing a first ply, by forming a first set of protuberances of limited size and high density on it;
embossing a second ply, by forming a second set of protuberances having larger sizes and a lower density than the first protuberances;
applying an adhesive between said two plies;
laminating the two plies together and joining them by gluing.
In practice, according to a particularly advantageous embodiment of the invention, the second ply is embossed between an embossing cylinder and a pressure cylinder having a yielding outer surface; the two embossed plies are laminated between the embossing cylinder and a laminating cylinder, whose outer surface has a greater hardness than that of said pressure cylinder.
Advantageously, the adhesive is applied to the extremities of at least some of the protuberances of said second set of protuberances formed on the second ply while said second ply is still engaged on the second embossing cylinder. To limit the quantity of adhesive applied, it is possible to arrange for it to be distributed on only some of said protuberances and not on all of them, for example by means of a patterned dispensing cylinder, or by using protuberances 9P of different heights.
By means of the method according to the invention, an embossed web material comprising at least two embossed plies joined together is obtained. A first of said two plies has a first set of protuberances, and the second of said two plies has a second set of protuberances facing the interior of the web material; the protuberances of the second set have larger sizes and a lower density than the protuberances of the first set, and the two plies are glued together at the positions of at least some of said second protuberances. The protuberances of the first set impart the technical characteristics of thickness, and therefore of volume, to the material, while the second protuberances provide the visual characteristics of the product, which can thus be personalized by maintaining a base pattern formed by the protuberances of the first set, and a motif, variable from one product to another, consisting of the set of protuberances of the second set.
Further advantageous features of the device, the method and the product according to the invention are indicated in the attached claims.
The presence of a micro-embossed ply, in other words one provided with protuberances having small sizes and a high density, typically equal to or greater than 30 points/cm2, makes it possible to obtain a product which, when wound in rolls, does not give rise to nesting between consecutive turns. The winding is carried out with the ply provided with the small protuberances facing the interior of the roll, while the protuberances of the second set, which form the decorative motif, face the exterior and are therefore clearly visible on the product.
Additionally, the micro-embossing imparts a high degree of plumpness and softness to the article, while the volume of the article is particularly high because of the presence of the embossing on the two plies and because of the joining of the two plies, which is essentially free of nesting.